Gate structure and method of fabricating the same

ABSTRACT

A gate structure includes at least one spacer defining a gate region over a semiconductor substrate, a gate dielectric layer disposed on the gate region over the semiconductor substrate, a first work function metal layer disposed over the gate dielectric layer and lining a bottom surface of an inner sidewall of the spacer, and a filling metal partially wrapped by the first work function metal layer. The filling metal includes a first portion and a second portion, wherein the first portion is between the second portion and the semiconductor substrate, and the second portion is wider than the first portion.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a continuation of and claims priority tocontinuation of and claims priority to U.S. Non-Provisional applicationSer. No. 16/015,392, titled “GATE STRUCTURE AND METHOD OF FABRICATINGTHE SAME” and filed on Jun. 22, 2018, which claims priority of U.S.Non-Provisional application Ser. No. 15/293,259, titled “GATE STRUCTUREAND METHOD OF FABRICATING THE SAME” and filed on Oct. 13, 2016, whichclaims priority of U.S. Provisional Application Ser. No. 62/261,201,titled “APPROACH OF MG PULL BACK FOR MG MISSING” and filed on Nov. 30,2015. U.S. Non-Provisional application Ser. No. 16/015,392, U.S.Non-Provisional application Ser. No. 15/293,259, and U.S. ProvisionalApplication Ser. No. 62/261,201 are incorporated herein by reference intheir entirety.

BACKGROUND

As technology nodes shrink, in some integrated circuit designs,replacing the polysilicon gate electrode with a metal gate electrode canimprove device performance with the decreased feature sizes. Providingmetal gate structures (e.g., including a metal gate electrode ratherthan polysilicon) offers one solution. One process of forming a metalgate stack is termed a “gate last” process in which the final gate stackis fabricated “last” which allows for a reduced number of subsequentprocesses, including high temperature processing, that are performedbefore formation of the gate stack. Additionally, as the dimensions oftransistors decrease, the thickness of the gate oxide may be reduced tomaintain performance with the decreased gate length. In order to reducegate leakage, high dielectric constant (high-k or HK) gate insulatorlayers are also used which allows to maintain the same effectivethickness as would be provided by a typical gate oxide used in largertechnology nodes.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a flowchart of a method of fabricating a gate structure inaccordance with some embodiments of the instant disclosure;

FIGS. 2 to 19 are cross-sectional views of a method for manufacturing asemiconductor device at various stages in accordance with someembodiments of the present disclosure;

FIGS. 20 and 21 are zoom in view of dashed-line circles in FIG. 17respectively; and

FIG. 22 illustrates the cross-sectional view of an intermediate stage inthe formation of a high-k metal gate stack in accordance with someembodiments of the instant disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

FIG. 1 is a flowchart of a method 100 of fabricating a gate structure inaccordance with some exemplary embodiments of the instant disclosure.The method begins with operation 101 in which a dummy gate layer stackis formed on a semiconductor substrate of a wafer. The method continueswith operation 103 in which vertical dummy gate stacks are formed bypatterning the dummy gate layer stack. Subsequently, operation 105,lightly-doped drain and source (LDD) regions are formed in thesemiconductor substrate. The method continues with operation 107 inwhich spacers are formed adjacent to the dummy gate stack. The methodcontinues with operation 109 in which source and drain regions areformed in the semiconductor substrate. The method continues withoperation 111 in which an inter-level dielectric (ILD) layer around thespacers. Next, operation 113, the dummy gate stacks are removed to formrecesses. The method continues with operation 115 in which work functionmetal layers are deposited in the recesses. Following that, operation117, a first portion of the work function metal layers from thesidewalls of the recesses is removed. In operation 119, a remainingportion of the recesses is filled with a filling metal. The methodcontinues with operation 121 in which portions of the filling metal andwork function metal layer is removed. Next, in operation 123, aremaining portion of the recesses is filled with a protection layer. Inoperation 125, the protection layer, the spacers, and the ILD layer areplanarized.

FIGS. 2 to 19 are cross-sectional views of a method for manufacturing asemiconductor device at various stages in accordance with someembodiments of the present disclosure. In FIGS. 2-19, the wafer 300 is asemiconductor device at an intermediate stage of manufacture. The wafer300 includes a semiconductor substrate 301. Examples of semiconductorsinclude silicon, silicon on insulator (SOI), Ge, SiC, GaAs, GaAlAs, InP,and GaNSiGe. The semiconductor substrate 301 may be doped of eithern-type or p-type, or undoped. Metal oxide semiconductor field effecttransistors (MOSFETs) are added to the wafer 300. These can be of then-type, the p-type or both types in a complementary metal oxidesemiconductor (CMOS) process. In some embodiments, the wafer 300includes n-well regions, p-well regions, or both. The method shown inFIGS. 1-19 is applicable to form planar MOSFETs and/or fin field effecttransistors (FinFETs). When the method shown in FIGS. 2-19 is applied toform FinFETs, the semiconductor substrate 301 includes at least one finstructure. The portion of the semiconductor substrate 301 shown in FIGS.2-19 is a portion of the fin structure.

Reference is made to FIG. 2. An interfacial layer 303 and a high-kdielectric layer (gate dielectric) 305 are formed over the semiconductorsubstrate 301 (operation 101 of FIG. 1). The interfacial layer 303 isthe interface between the semiconductor substrate 301 and the high-kdielectric layer (gate dielectric) 305. The interfacial layer 303includes silicon oxide or silicon oxynitride. The interfacial layer 303can form spontaneously as a result of wet cleans of the wafer 300 priorto the formation of the high-k dielectric layer 305 or as a result ofinteraction between the high-k dielectric layer 305 and thesemiconductor substrate 301 during or subsequent to formation of thedielectric layer 305. Intentionally forming the interfacial layer 303can provide a higher quality interface. The interfacial layer 303 ismade very thin to minimize the interfacial layer's contribution to theoverall equivalent oxide thickness of the resulting gates. In someembodiments, the thickness of the interfacial layer 303 is in a rangefrom about 1 to about 20 Angstroms.

The interfacial layer 303 of silicon oxide can be formed by a suitableprocess including chemical oxidation, for example, by treating thesemiconductor substrate 301 with hydrofluoric acid (HF) immediatelyprior to depositing the high-k dielectric layer 305. Another process forthe silicon oxide interfacial layer 303 is to thermally grow theinterfacial layer 303 followed by a controlled etch to provide thedesired layer thickness. In some embodiments, the interfacial layer 303can be formed after the high-k dielectric layer 305. For example, asilicon oxynitride interfacial layer can be formed by annealing a waferwith a silicon semiconductor substrate and a hafnium-based high-kdielectric layer in an atmosphere of nitric oxide. This later processhas advantages such as reduced queue time.

The high-k dielectric layer 305 includes one or more layers of one ormore high-k dielectric materials. High-k dielectrics are expected tohave a dielectric constant, k, of at least or equal to about 4.0.Examples of high-k dielectrics include hafnium-based materials such asHfO₂, HfSiO, HfSiON, HfTaO, HfTiO, HfZrO, and HfO₂Al₂O₃ alloy.Additional examples of high-k dielectrics include ZrO₂, Ta₂O₅, Al₂O₃,Y₂O₃, La₂O₃, and SrTiO₃. In some embodiments, the high-k dielectriclayer 305 has a thickness in a range from about 5 to about 50 Angstroms.The high-k dielectric layer 305 can be formed by, for example, chemicalvapor deposition (CVD) or atomic layer deposition (ALD).

Optionally, a capping layer may be formed over the high-k dielectriclayer 305. The capping layer can protect the high-k dielectric layer 305during subsequent processing and provide an etch stop layer for when thedummy gate material layer 307 is later removed. The capping layer caninclude one or more layers of materials, which may include, for example,TiN and TaN. The capping layer can be formed by a deposition process,such as CVD, ALD, or electroplating to a specified thickness.

Still referring to FIG. 2, a dummy gate material layer 307 is formedover the high-k dielectric layer 305. The dummy gate material layer 307is made of polysilicon, although other materials can be used. The dummygate material layer 307 can be formed by a semiconductor depositionprocess. For example, a polysilicon dummy gate material layer can beformed by pyrolyzing silane. After formation of the dummy gate materiallayer 307, a dummy gate layer stack 310 is formed on the wafer 300 asshown in FIG. 1. The dummy gate layer stack 310 includes the interfaciallayer 303, the high-k dielectric layer 305 and the dummy gate materiallayer 307.

Reference is made to FIGS. 3-4. The dummy gate layer stack 310 ispatterned to form dummy gate stacks 318 a and 318 b (operation 103 ofFIG. 1). For forming the dummy gate stacks 318 a and 318 b, thepatterning can be accomplished by a photolithographic process. Thephotolithography process includes coating the wafer 300 with aphotoresist, selectively exposing the photoresist according to a desiredpattern, developing the photoresist, and using the patterned photoresistas an etch mask. The patterned photoresist can be used as a mask to etchthe dummy gate layer stack 310. Alternatively, the photoresist is usedto pattern a hard mask layer. The hard mask layer, if used, is formedbefore the photoresist. The wafer 300 of FIG. 1 includes a hard masklayer 309 before patterning. The wafer 300 of FIG. 2 includes thepatterned hard mask layers 309 a and 309 b. The patterned hard masklayers 309 a and 309 b are used as masks to etch the dummy gate layerstack 310. Any etch process or combination of etch processes can be usedto etch the dummy gate layer stack 310.

Reference is made to FIG. 4. After patterning the dummy gate layer stack310, the dummy gate stacks 318 a and 318 b are formed. The dummy gate318 a includes the interfacial layer 303 a, the high-k dielectric layer305 a, the dummy gate material layer 307 a, and the patterned hard masklayer 309 a. Likewise, the dummy gate 318 b includes the interfaciallayer 303 b, the high-k dielectric layer 305 b, the dummy gate materiallayer 307 b, and the patterned hard mask layer 309 b. It should beunderstood that the dummy gate stacks 318 a and 318 b may not beadjacent to each other. For the sake of clarity and simplicity, the twodummy gate stacks 318 a and 318 b are put together for illustrationpurpose. The dummy gate stacks 318 a and 318 b may be separated apart byother features not shown in the figure.

A process for etching the dummy gate layer stack 310 includes a plasmaetch. Reactive gases can interact with the wafer 300 during plasmaetching to produce volatile by products that subsequently redeposit onnearby surfaces. This can result in the formation of an optionalpassivation layer (not shown) on sidewalls of the dummy gate stacks 318a and 318 b respectively. The optional passivation layers can be silicaor a similar material such as a silicate.

An ion implantation process is performed to form lightly doped drain(LDD) regions (operation 105 of FIG. 1). The dummy gate stacks 318 a and318 b are used as masks to help control the implant profile anddistribution. FIG. 5 shows the wafer 300 with the LDD regions 329 a and329 b formed in the semiconductor substrate 301. After the ionimplantation process, spacers 320 a and 320 b are formed around thedummy gate stacks 318 a and 318 b (operation 107 of FIG. 1). A spacermaterial is first deposited over the wafer 300 covering the dummy gatestacks 318 a and 318 b and the areas between the dummy gate stacks 318 aand 318 b. The spacer material is then etched back to remove theportions over the dummy gate stacks 318 a and 318 b and in the areasbetween the dummy gate stacks 318 a, 318 b. By tuning the etch process,selected portions 320 a and 320 b of the spacer material around thedummy gate stacks 318 a and 318 b remain after the etch back.

Before forming the spacers, optional spacer liners (not shown) may beformed. The spacer liners may be silica or silicate. The material of thespacer liners can be similar to the material of the passivation layersif both layers are present. The spacers 320 a and 320 b may be made ofsilicon nitride or another material that has the properties of conformaldeposition, a large etch selectivity against the dummy gate material(harder to etch than the dummy gate material) and a passive materialthat can trap implanted dopants.

Still referring to FIG. 5, source/drain regions 327 a and 327 b areformed after the spacers 320 a and 320 b are formed (operation 109 ofFIG. 1). The source/drain regions 327 a and 327 b are formed in thesemiconductor substrate 301. In the embodiments where the dummy gatestack 318 a and/or the dummy gate 318 b is used to form a p-channelmetal oxide semiconductor field effect transistor (pMOS) device, thesource/drain regions 327 a and/or the source/drain regions 327 b are ofp-type. In the embodiments where the dummy gate stack 318 a and/or thedummy gate 318 b is used to form an n-channel metal oxide semiconductorfield effect transistor (nMOS) device, the source/drain regions 327 aand/or the source/drain regions 327 b are of n-type. The formation ofsource/drain regions 327 a and 327 b may be achieved by etching thesemiconductor substrate 301 to form recesses therein, and thenperforming an epitaxy to grow the source/drain regions 327 a and 327 bin the recesses.

An inter-level dielectric (ILD) layer 319 is formed, as illustrated inFIG. 6 (operation 111 of FIG. 1). The ILD layer 319 adheres well to thespacers 320 a and 320 b and over the top of the hard mask layers 309 aand 309 b.

Reference is made to FIG. 7. After the ILD layer 319 is formed, an uppersurface of the wafer 300 is planarized to lower the surface to the levelof the dummy gate material layers 307 a and 307 b. The planarization isaccomplished by, for example, chemical mechanical polishing (CMP). Afterplanarizing, the patterned hard mask layers 309 a and 309 b are removed,and the dummy gate material layers 307 a and 307 b, the spacers 320 aand 320 b, and the ILD layer 319 all approximately have the same height.

Reference is made to FIG. 8. The dummy gate material layers 307 a and307 b are removed to form recesses 312 a and 312 b (operation 113 ofFIG. 1). The dummy gate material layers 307 a and 307 b are removed inone or many etch operations including wet etch and dry etch. Accordingto various embodiments, a hard mask is patterned over the wafer 300 toprotect the ILD layer 319 and the spacers 320 a and 320 b. In someembodiments, a first etch process breaks through native oxide layers onthe dummy gate material layers 307 a and 307 b, and a second etchprocess reduces the thickness of the dummy gate material layers 307 aand 307 b. The dummy gate material layer etch may stop at the high-kdielectric layers 305 a and 305 b or continues to the interfacial layers303 a and 303 b or the semiconductor substrate 301 below. In otherembodiments, only the dummy gate material layers 307 a and 307 b areremoved. However, the etch processes may remove some surroundingmaterial such as a portion of the spacers 320 a and 320 b. A recess 312a is formed between the spacers 320 a, and a recess 312 b is formedbetween the spacers 320 b. As previously discussed, the high-kdielectric layers 305 a, 305 b may also be removed. If it is, then ahigh-k dielectric layer is formed in the recesses in a separateoperation.

Attention is now invited to FIG. 9. A plurality of work function metallayers is deposited in the recesses 312 a and 312 b (operation 115 ofFIG. 1). Two gate structures are denoted as 300 a and 300 b respectivelyfor ease of reference. A first work function metal layer 330 is formedin the recesses 312 a and 312 b and follows the contour created bybottom surfaces and sidewalls of the recesses 312 a and 312 b and topsurfaces of the spacers 320 a and 320 b and the ILD layer 319. A secondwork function metal layer 340 is deposited on the first work functionmetal layer 330 and conforms to the first work function metal layer 330.The first work function metal layer 330 is in direct contact with thehigh-k dielectric layers 305 a and 305 b. The second work function metallayer 340 inherits the configuration of the first work function metallayer 330.

The first and second work function metal layers 330 and 340 may includeTi, TiAl, TiAlN, TaC, TaCN, TaSiN, Mn, Zr, TiN, TaN, Ru, Mo, WN, Co, Al,or any suitable materials. For example, the first and second workfunction metal layers 330 and 340 include at least one of TiN, Co, WN,or TaC when at least one of the gate structures 300 a and 300 b is aportion of a PMOS device. Alternatively, the first and second workfunction metal layers 330 and 340 include at least one of Ti, Al, orTiAl when at least one of the gate structures 330 a and 300 b is aportion of an NMOS device. The first and second work function metallayers 330 and 330 may be deposited by, for example, CVD,plasma-enhanced CVD (PECVD), sputtering, ion beam, spin on, physicalvapor deposition (PVD), ALD or the like.

Attention is now invited to FIGS. 10-13. The second work function metallayer 340 is pulled back in two stages (operation 117 of FIG. 1). Asshown in FIG. 10, a mask layer 345 is deposited on the substrate 301. Insome embodiments, the mask layer 345 is, for example, a bottomanti-reflective coating (BARC) layer. The mask layer 345 fills in therecesses 312 a and 312 b and covers up the entire second work functionmeta layer 340 on the top surfaces of the spacers 320 a and 320 b andthe ILD layer 319.

Next, please refer to FIG. 11. A first etching, for example, dryetching, is performed to pattern the mask layer 345. The patterned masklayers 345 a and 345 b retreats into the recesses 312 a and 312 brespectively. A surface level of the mask layers 345 a and 345 b iswithin the recesses 312 a and 312 b.

Attention is now invited to FIG. 12. After the first etching, in whichthe mask layer 345 is patterned to form the mask layers 345 a and 345 b,a second etching is performed. The second etching, for example, wetetching, targets at the second work function metal layer 340. During thesecond etching, the patterned mask layers 345 a and 345 b protect theunderlying second work function metal layer 340 in the recesses 312 aand 312 b. After the second etching, the second work function metallayers 340 a and 340 b are lowered respectively into the recesses 312 aand 312 b, and top edges of the second work function metal layers 340 aand 340 b are modified along the course of the second etching to formslanting edges 342 a and 342 b. In some embodiments, the first workfunction metal layer 330 and the second work function metal layer 340are made of different materials. The first work function metal layer 330is made of a material that has an etch selectivity against the secondwork function metal layer 340 during the second etching.

Attention is now invited to FIG. 13. The pull-back process targets atthe second work function metal layer 340, while the first work functionmetal layer 330 retains its integrity at this stage because of etchselectivity. The patterned mask layers 345 a and 345 b are then removedfrom the wafer 300. The slanting edges 342 a and 342 b have a slopedescending inwardly toward the respective recesses 312 a and 312 b (awayfrom the spacers 320 a and 320 b). The slope of the tapered, slantingedges 342 a and 342 b ranges from about 15 to about 45 degrees. In someembodiments, the slanting edges 342 a and 342 b are rounded corners.

Turning now to FIG. 14. A third work function metal layer 350 isdeposited on the wafer 300, and then a portion of the third workfunction metal layer 350 in the gate structure 300 a is removed. In someembodiments, the gate structure 300 a and the gate structure 300 b areused to form transistors with different threshold voltages ortransistors of different types, and, therefore, the gate structure 300 aand the gate structure 300 b have different numbers of work functionmetal layers. In some embodiments, the gate structure 300 a does notinclude the third work function metal layer 350, while the gatestructure 300 b includes the third work function metal layer 350. Insome embodiments, the gate structure 300 a is a p-type gate electrode,and the gate structure 300 b is an n-type gate electrode. A designedthreshold voltage for n-type and p-type devices can be tuned throughdifferent combination of work function metal layers. Different patternsarise between the gate structures 300 a and 300 b because of differentnumbers or combination of work function metal layers so as to achievedesired threshold voltage. The third work function metal layer 350conforms to the padded recess 312 b, where the first work function metallayer 330 and the second work function metal layer 340 b line the bottomsurface and the sidewalls of the recess 312 b. The third work functionmetal layer 350 overtakes the slanting edges 342 b of the second workfunction metal layer 340 b in the recess 312 b, and therefore both thesecond and third work function metal layers 340 b and 350 are in contactwith the first work function metal layer 330. In addition, because thesecond work function metal layer 340 b is modified at the edges, thethird work function metal layer 350 follows the inverted, stepped,pyramid topology in the recess 312 b.

Turning now to FIG. 15, the third work function metal layer 350 ispulled back to yield slanting edges 352. Similar to the second workfunction metal layers 340 a and 340 b, the third work function metallayer 350 undergoes a series of etching. A mask layer is deposited, andthe first etching defines a patterned mask layer over the third workfunction metal layer 350 in the recess 312 b. Subsequently, the secondetching results in the receding of the third work function metal layer350 within the recess 312 b and formation of slanting edges 352. Thethird work function metal layer 350 covers up the underlying second workfunction metal layer 340 b. After the second etching, the third workfunction metal layer 350 contributes another level to the slantingsidewalls of the recess 312 b. The slanting edges 342 b are translatedinto the third work function metal layer 350. Likewise, the pull-backprocess targets at specific work function metal layer because of etchselectivity, and the first work function metal layer 330 retains itsintegrity throughout the second and third work function metal layerpull-back.

Turning now to FIG. 16, a filling metal 360 is deposited over the wafer300 (operation 119 of FIG. 1). The filling metal 360 fills in theremaining portion of the recesses 312 a and 312 b and overfills therecesses 312 a and 312 b to cover up the first work function meta layer330 on the top surfaces of the spacers 320 a and 320 b and the ILD layer319. A material of the filling metal 360 may include, for example,tungsten (W). The gate structures 300 a and 300 b have differentpatterns results from different numbers of work function metal layers.As shown in FIG. 16, after the deposition of filling metal 360, thedifferent contour of the recesses 312 a and 312 b is more pronounced. Inthe gate structure 300 a, a portion of the filling metal 360 is enclosedby the second work function metal layer 340 a, while the remainingportion of the filling metal 360 is in contact with the first workfunction metal layer 330. In the gate structure 300 b, the filling metal360 overfills the recess 312 b and blankets the first work functionmetal layer 330 and the third work function metal layer 350. The fillingmetal 360 in the recess 312 b is not in direct contact with the secondwork function metal layer 340 b because the second work function metallayer 340 b underlies the third work function metal layer 350 and isunexposed. The second work function metal layer 340 b still contributesto the topology of the stepped recess 312 b and serves its intendedfunction, voltage manipulation. The tripled work function metal layers330, 340 b, and 350 collectively create tapered sidewalls in the recess312 b with an additional level in comparison with the doubled workfunction metal layers 330 and 340 a in the recess 312 a.

Attention is now invited to FIG. 17. An etching back is performed tobring down the filling metal 360 and the first work function metal layer330 within the recesses 312 a and 312 b (operation 121 of FIG. 1). Auniversal etching back does not take different patterns in gatestructures into consideration. When only filling metal 360 and the firstwork function metal layers 330 are the targets in the etching backprocess, variation between the gate structures is minimized.

Still referring to FIG. 17, the filling metal 360 a is lowered to alevel within the recesses 312 a and 312 b respectively, and the firstwork function metal layer 330 over the inter-level dielectric layer 319is removed. The etching back continues until the first work functionmetal layers 330 a and 330 b retreat into the recess 312 a and 312 b.Slanting edges 332 a and 332 b of the first work function metal layers330 a and 330 b respectively are formed during the etching back. Thefilling metals 360 a and 360 b reach to the brim of the first workfunction metal layers 330 a and 330 b in the recesses 312 a and 312 b.As a result, the filling metals 360 a and 360 b have surface areas thatare both defined by the first work function metal layers 330 a and 330b. In the universal etching back, only the first work function metallayers 330 and the filling metal 360 are removed. Because the second andthe third work function metal layers 340 a, 340 b, and 350 are buriedunderneath in the recesses 312 a and 312 b respectively. When performingthe etching back, the loading pattern arising from different numbers ofwork function metal layers can be omitted.

Turning now to FIGS. 20 and 21, illustrated zoom in view of the gatestructures 300 a and 300 b. When the universal etching back is performedto lower the surface level of the filling metal 360, the filling metals360 a and 360 b are brought to the same level within their respectiverecesses 312 a and 312 b. Portions of the first work function metallayers 330 are removed in the etching back, while the second and thirdwork function metal layers 340 a and 350 retain their configuration andare unexposed. A lower portion of the filling metals 360 a and 360 b arein contact with the second work function metal layer 340 a or the thirdwork function metal layer 350. The work function metal layers 330 a, 330b, 340 a, 340 b, and 350 serve their intended function, while the workfunction metal layers 340 a, 340 b, and 350 is sealed behind the fillingmetals 360 a and 360 b. Since the second and third work function metallayers 340 a, 340 b, and 350 are buried under the filling metals 360 aand 360 b, even if the loading patterns (i.e., numbers of work functionmetal layers) in the gate structures 300 a and 300 b are different, thetopology from a top view is similar.

As shown in FIG. 20, the first and second work function metal layers 330a and 340 a create tapered sidewalls in the recesses 312 a. The fillingmetal 360 a fills in the recess 312 a and resembles a two-level invertedpyramid. The first work function metal layer 330 a defines a first widthW₁ that is measured from one slanting edge 332 to the other slantingedge 332. The second work function metal layer 340 a defines a secondwidth W₂ that is measured from one slanting edges 342 a to the otherslanting edge 342 a. The filling metal 360 a fills in the tapered recess312 a, and a first portion 361 a of the filling metal 360 a is betweenthe semiconductor substrate 301 and a second portion 362 a of thefilling metal 360 a. The first portion 361 a of the filling metal 360 ahas the second width W₂, and a second portion 362 a of the filling metal360 a has the first width W₁. The filling metal 360 a fans out from thebottom surface of the recess 312 a because the second work functionmetal layer 340 a is buried underneath. The broader first width W₁ isretained for the filling metal 360 a, and the second work function metallayer 340 a along with its narrower second width W₂ is unexposed.

Likewise, as shown in FIG. 21, in addition to the first and second widthW₁ and W₂ which are defined by the first and second work function metallayers 330 b and 340 b respectively, the third work function metal layer350 defines a third width W₃ that is measured from one slanting edges352 to the other slanting edge 352. The third width W₃ is the narrowestamong the three widths because the third work function metal layer 350is further compressed within the space left out by the second workfunction metal layer 340 b. The filling metal 360 b fills in the taperedrecess 312 b, and from the bottom to the top are the first portion 361b, the second portion 362 b and a third portion 363 b. The third portion363 b of the filling metal 360 b has the broadest first width W₁. Thefilling metal 360 a fans out from the bottom surface of the recess 312 bbecause the second and the third work function metal layers 340 b and350 are buried under the filling metal 360 b. The broader third portion363 b of the filling metal, which has the first width W₁, is retained,and the narrower first and second portions 361 b and 362 b of thefilling metal 360 b are buried underneath.

In practical, on top of the width of each of the work function metallayers, the work function metal layers have varied slopes along thesidewalls of the recess. In the recess 312 a shown in FIG. 20, the firstwork function metal layer 330 a has a milder slope in comparison withthe second work function metal layer 340 a. The second work functionmetal layer 340 a has a nearly vertical slope at the bottom of therecess 312 a. The first portion 361 a of the filling metal 360 a, whichfills in the bottom portion of the recess 312 a, has a steeper slopethan the second portion 362 a thereof. As shown in FIG. 21, the thirdwork function metal layer 350 adds another level to the taperedsidewalls of the recess 312 b, and the slopes from the top to the bottomsurface of the recess 312 b increases gradually. The first portion 361 bof the filling metal 360 b has a nearly vertical slope at the bottom ofthe recess 312 b, and when it comes to the second portion 362 b of thefilling metal 360 b, the slope becomes milder. The third portion 363 bof the filling metal 360 b, which is at the top portion of the recess312 b, has the least steep slope in the recess 312 b.

Still referring to FIGS. 20 and 21, regardless the element arrangementwithin the recesses 312 a and 312 b, a top view of the gate structures300 a and 300 b is similar with only the filling metals 360 a and 360 band the first work function metal layer 330 a and 330 b to be found.More than similar exposed elements in different gate structures, theconfiguration form the top view is uniform as well. The filling metals360 a and 360 b have the same width which is defined by the lip portionof the first work function metal layers 330 a and 330 b respectively.The uniform topology of the gate structures 300 a and 300 b from the topview have advantageous effects to the subsequent process.

Turning now to FIG. 18, a protection layer 370, for example, a nitridelayer, fills in the remaining of the recesses 312 a and 312 b. Theprotection layer 370 serves to protect the underlying components likethe work function metal layers. In either the recess 312 a or the recess312 b, the protection layer 370 is held at the same level. In addition,the underlying element arrangement is uniform. The protection layer 370is in contact with the slanting edges 332 a and 332 b of the first workfunction metal layer 330 a and 330 b and the filling metals 360 a and360 b. The slanting edges 332 a and 332 b of the first work functionmetal layer 330 a and 330 b are at the same height, and the fillingmetals 360 a and 360 b have the same surface area and dimension from atop view.

Turning now to FIG. 19, a polishing process, for example, CMP isperformed, and the gate structures 300 a and 300 b are lowered to alevel near to the slanting edges 332 a and 332 b of the first workfunction metal layers 330 a and 330 b. Due to the same topology withinthe recesses 312 a and 312 b, the position of the slanting edges 332 aand 332 b are taken into consideration. That is, regardless the numberof work function metal layers, the protection layer 370 polishing isuniversally applied to the gate structures 300 a and 300 b with the sameparameters because the interface topology between the protection layer370 and in each of the gate structures 300 a and 300 b are similar, andthe interface are located at the same level. In this case, edges of thework function metal layers are less likely to go through the protectionlayers 370 a and 370 b in the polishing process.

The protection layers 370 a and 370 b prevent aggressive invasion, forexample, chemical agent like acid in the following etching process. Inthe case when defects are formed in the protection layer, foreignmaterial can cause metal gate missing or compromising the function ofother components. By having the same topology even with differentloading patterns, when polishing the protection layer, attention is paidto the first work function metal layer and the filling metal withoutworrying the underlying work function metal layers in different gatestructures.

Turning now to FIG. 22, a gate structure 300 c is shown with four-layerof work function metal layers. The gate structure 300 c includes thefirst, second, third work function metal layers 330 c, 340 c, and 350 c.In addition, the gate structure 300 c includes a fourth work functionmetal layer 380 formed over the third work function metal layer 350 c.Compared to the gate structure 300 b, the gate structure 300 c goesthrough one more work function metal layer pull-back in the process. Thefourth work function metal layer 380 blankets the third work functionmetal layer 350 c within the recess 312 c, and the slanting edges 342 cand 352 c are translated into the fourth work function metal layer 380.The sidewalls of the recess 312 c show a four-level inverted pyramidwith gradually reduced slope from the bottom to the top. The fillingmetal 360 c still has the same surface area and topology with thefilling metals 360 a and 360 b even if the number of work function metallayers increases to four.

Apart from the first work function metal layers, the remaining workfunction metal layers are buried under the filling metal. Etching backof the first work function metal layer and the filling metal will bemuch easier because other than the first work function metal layer theremaining work function metal layers are not etched during the etchingback. The resulting configuration gives similar topology from a top viewamong different gate structures.

In some embodiments of the instant disclosure, a gate structure includesat least one spacer defining a gate region over a semiconductorsubstrate, a gate dielectric layer disposed on the gate region over thesemiconductor substrate, a first work function metal layer disposed overthe gate dielectric layer and lining a bottom surface of an innersidewall of the spacer, and a filling metal partially wrapped by thefirst work function metal layer. The filling metal includes a firstportion and a second portion, wherein the first portion is between thesecond portion and the substrate, and the second portion is wider thanthe first portion.

In some embodiments of the instant disclosure, a gate structure includesat least one spacer defining a gate region over a substrate, a gatedielectric layer disposed on the gate region over the substrate, a firstwork function metal layer disposed over the gate dielectric layer andlining portions of an inner sidewall of the spacer. The first workfunction metal layer has at least on slanting edge. The gate structurealso includes a filling metal partially wrapped by the first workfunction metal layer. The slanting edge of the first work function metallayer is buried under the filling metal.

In some embodiments of the instant disclosure, a method includes formingat least one dummy gate stack including a gate dielectric layer and adummy gate material layer overlying the gate dielectric layer. Aninter-layer dielectric (ILD) layer is formed around the dummy gatestack. At least the dummy gate material layer is removed from the dummygate stack to form at least one recess. At least one work function metallayer is formed on a bottom surface and at least one sidewall of therecess. A first portion of the work function metal layer is removed fromthe sidewall of the recess. A second portion of the work function metallayer remains on the sidewall of the recess after the removing. Then, aremaining portion of the recess is filled with a filling metal.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the instantdisclosure. Those skilled in the art should appreciate that they mayreadily use the instant disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the instantdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of theinstant disclosure.

What is claimed is:
 1. A gate structure, comprising: a first workfunction metal layer disposed over a gate dielectric layer; a secondwork function metal layer disposed over the first work function metallayer; and a filling metal, wherein: a first portion of a sidewall ofthe filling metal is over the second work function metal layer andlaterally adjacent the first work function metal layer, a second portionof the sidewall of the filling metal is laterally adjacent the secondwork function metal layer, and the first portion of the sidewall has afirst slope and the second portion of the sidewall has a second slopedifferent than the first slope.
 2. The gate structure of claim 1,wherein the second slope is greater than the first slope.
 3. The gatestructure of claim 1, wherein the second portion of the sidewall of thefilling metal is laterally adjacent the first work function metal layer.4. The gate structure of claim 3, wherein the second portion of thesidewall of the filling metal is spaced apart from the first workfunction metal layer by the second work function metal layer.
 5. Thegate structure of claim 1, comprising: a third work function metal layerdisposed over the second work function metal layer, wherein: a thirdportion of the sidewall is over the second work function metal layer andbetween the first portion of the sidewall and the second portion of thesidewall, the third portion of the sidewall is laterally adjacent thethird work function metal layer, and the third portion of the sidewallhas a third slope different than the second slope and different than thefirst slope.
 6. The gate structure of claim 5, wherein the second slopeis greater than the third slope and the third slope is greater than thefirst slope.
 7. The gate structure of claim 1, comprising a protectionlayer over the filing metal, wherein the protection layer is spacedapart from the second work function metal layer by the filling metal. 8.The gate structure of claim 7, wherein the protection layer is incontact with the first work function metal layer.
 9. The gate structureof claim 1, wherein: the first work function metal layer is disposedbetween a first spacer and a second spacer, the gate structure comprisesa protection layer extending from the first spacer to the second spacer,and the protection layer is in contact with first work function metallayer.
 10. The gate structure of claim 9, wherein the protection layeris in contact with the first spacer.
 11. A gate structure, comprising: afirst work function metal layer disposed over a gate dielectric layer; asecond work function metal layer disposed over the first work functionmetal layer, wherein the second work function metal layer has a slopedtop surface; and a filling metal, wherein the second work function metallayer is spaced apart from the filling metal.
 12. The gate structure ofclaim 11, comprising: a protection layer over the filing metal, whereinthe protection layer is spaced apart from the second work function metallayer by the filling metal.
 13. The gate structure of claim 12, whereinthe protection layer is laterally adjacent the first work function metallayer.
 14. The gate structure of claim 12, wherein the protection layeris in contact with the first work function metal layer.
 15. The gatestructure of claim 11, wherein the filling metal is in contact with thefirst work function metal layer.
 16. The gate structure of claim 11,comprising: a third work function metal layer between the filling metaland the second work function metal layer.
 17. A method of forming a gatestructure, the method comprising: forming a first work function metallayer in a recess; forming a second work function metal layer in therecess over the first work function metal layer; removing a firstportion of the second work function metal layer; forming a filling metalin the recess after removing the first portion of the second workfunction metal layer; and forming a protection layer in the recess overthe filling metal, wherein the protection layer is spaced apart from asecond portion of the second work function metal layer by the fillingmetal.
 18. The method of claim 17, wherein removing the first portion ofthe second work function metal layer comprises removing the firstportion of the second work function metal layer to expose a firstportion of a sidewall of the first work function metal layer.
 19. Themethod of claim 18, wherein forming the protection layer comprisesforming the protection layer laterally adjacent the first portion of thesidewall of the first work function metal layer.
 20. The method of claim17, comprising etching back the filling metal before forming theprotection layer.